Many physical processes are inherently exothermic, meaning that some energy previously present in another form is converted to heat by the process. While the creation of heat energy may be the desired outcome of such a process, as with a boiler installed to provide radiant heat to a building using a network of conduits which circulate hot water to radiators or a furnace used for the smelting of metals, in many other instances unwanted heat is produced as a byproduct of the primary process. One such example is an automobile internal combustion engine, which provides motive force as well as significant unwanted heat. Even in those processes in which the generation of heat energy is desired, some degree of residual heat typically escapes or remains that can be managed and/or dissipated. Whether generated intentionally or incidentally, this residual or waste heat represents that portion of the input energy which was not successfully applied to the primary function of the process in question. This wasted energy detracts from the performance, efficiency, and cost effectiveness of the system.
With respect to the internal combustion engine (“ICE”), considerable waste heat energy is generated by the combustion of fuel and the friction of moving parts within the engine. ICE efficiency is generally less than 40%; 60% or more of the engine fuel's energy is therefore converted to waste heat energy that is commonly dissipated to the ICE's surroundings.
Automobiles are usually equipped with extensive systems that transfer the heat energy away from the source locations and distribute that energy throughout a closed-loop recirculating system. This recirculating system usually employs a water-based coolant medium flowing under pressure through jackets within the engine coupled to a radiator across which the imposition of forced air dissipates a portion of the undesired heat energy into the environment. This cooling system is managed to permit the engine to operate at the desired temperature, removing some but not all of the heat energy generated by the engine.
As a secondary function, a portion of the heat energy captured by the engine cooling system may be used to indirectly provide warm air as desired to the passenger compartment for the operator's comfort. This recaptured and re-tasked portion of the waste heat energy generated as a byproduct of the engine's primary function represents one familiar example of the beneficial use of waste heat.
Considerable additional waste heat is expelled from the ICE via the engine exhaust system. The byproducts of the combustion, including gasses containing some particulate matter, exit the engine as a result of the pressure differential between the engine's internal pressure and the lower ambient pressure. Considerable heat is also removed from the system in this process. For most ICE applications, however, it is uncommon to use the heat of the engine exhaust system for a secondary purpose. The temperature of the exhaust flow usually exceeds that of the cooling jacket water. However, the proportion of heat energy removed from the engine and/or available for conversion to other purposes via may not be similarly distributed. For example, the total available heat energy in the jacket water may be less than, equal to, or greater than the total heat energy contained in the exhaust gas flow.
In addition to the cooling of ICEs, jacket water cooling systems have been utilized in a number of other industrial applications, including but not limited to compressor heads or other components in which an increase in pressure, internal friction, or other physical phenomena causes an increase in temperature that must be removed from the system for proper operation. In such systems, exhaust gasses may simultaneously be generated by the same device or by an interconnected device or system, such as the source of power for a gas compressor system. In the case of systems that capture radiated energy including but not limited to solar-based systems, jacket water may be used to cool the apparatus. In some cases, this jacket cooling may be in addition to any primary flow of media inside the system that constitutes the primary conversion function of the system, and the heat energy captured by the secondary cooling system may be considered waste heat energy if it is of no use to the primary solar-based system.
Characteristics of the heat sources that affect quality may include but are not limited to its temperature (sufficiency and stability), form (gaseous, liquid, radiant, etc.), the presence of corrosive elements associated with the heat source, accessibility for use, and the duty cycle of availability. Waste heat energy sources are classified by grade according to these characteristics. Prior art ORC systems prefer higher grade sources of heat that are readily accessible, of generally high and stable temperature, are free of contaminants, and are available without interruption. Lower grade sources of heat, particularly those at lower temperatures, are not as desirable and have not been fully utilized by the prior art.
Large internal combustion engines, as another example, are widely used in heavy industry in numerous applications. For example, General Electric's Jenbacher gas engine division produces a full range of engines with output power capabilities ranging from 250 kW to over 8,000 kW. By comparison, a typical mid-class automobile engine produces about 150 kW of usable output power. The Jenbacher engines may be powered by a variety of fuels, including but not limited to diesel, gasoline, natural gas, biogas, and other combustible gasses including but not limited to those produced from landfills, sewage, and coal mines. These engines are frequently employed to drive electric power generators, thereby converting the mechanical energy produced from the energy of combustion into electrical energy.
In operation, these Jenbacher engines generate tremendous amounts of waste heat energy that has historically been dissipated into the environment. In the case of the combined Jenbacher model J316 engine and generator system with a rated electric power output of approximately 835 kW, approximately 460 kW of heat energy is lost (dissipated) in the exhaust gas at an approximate temperature of 950° F. and approximately another 570 kW is lost in the internal cooling system with a typical jacket water coolant temperature of approximately 200° F. Of that 570 kW, approximately 463 kW is suitable for waste heat recovery at sufficient temperature with the remainder of such low grade as to not be practicable for direct conversion. From this data, less than half of the system's energy output is in the desired form (in this case, electric power output from the system generator). In many prior art systems, a substantial portion of the input energy converted to heat will be lost The heat from exhaust gas generally escapes into the atmosphere, and the recirculating jacket water is cooled by an outboard apparatus (such as by large external condensing radiators driven by forced air sources), which consume additional electric power to function and further reduce the efficiency of the system.
Additionally, the dissipation of this waste heat energy into the environment can have deleterious effects. Localized heating may adversely affect local fauna and flora and can require additional power, either generated locally or purchased commercially, to provide additional or specialized cooling. Further, the noise generated by forced air cooling of the jacket water heat radiators can have undesirable secondary effects.
Waste heat energy systems employing the organic Rankine cycle (ORC) system have been developed and employed to recapture waste heat from sources such as the Jenbacher 312 and 316 combustion engines. One typical prior art ORC system for electric power generation from waste heat is depicted in FIG. 1. Heat exchanger 101 receives a flow of a heat exchange medium in a closed loop system heated by energy from a large internal combustion engine at port 106.
For example, this heat energy may be directly supplied from the combustion engine via the jacket water heated when cooling the combustion engine, or it may be coupled to the ORC system via an intermediate heat exchanger system installed proximate to the source of exhaust gas of one or more combustion engines. In either event, heated matter from the combustion engine or heat exchanger is pumped to port 106 or its dedicated equivalent. The heated matter flows through heat exchanger 101 and exits at port 107 after transferring a portion of its latent heat energy to the separate but thermally coupled closed loop ORC system which typically employs an organic refrigerant as a working fluid. Under pressure from the system pump 105, the heated working fluid, predominantly in a gaseous state, is applied to the input port of expander 102, which may be a positive displacement machine of various configurations, including but not limited to a twin screw expander or a turbine. Here, the heated and pressurized working fluid is allowed to expand within the device, and such expansion produces rotational kinetic energy that is operatively coupled to drive electrical generator 103 and produce electric power which then may be delivered to a local, isolated power grid or the commercial power grid. The expanded working fluid at the output port of the expander, which typically is a mixture of liquid and gaseous working fluid, is then delivered to condenser subsystem 104 where it is cooled until it has returned to its fully liquid state.
The condenser subsystem sometimes includes an array of air-cooler radiators or another system of equivalent performance through which the working fluid is circulated until it reaches the desired temperature and state, at which point it is applied to the input of system pump 105. System pump 105 provides the motive force to pressurize the entire system and supply the liquid working fluid to heat exchanger 101, where it once again is heated by the energy supplied by the combustion engine waste heat and experiences a phase change to its gaseous state as the organic Rankine cycle repeats. The presence of working fluid throughout the closed loop system ensures that the process is continuous as long as sufficient heat energy is present at input port 106 to provide the requisite energy to heat the working fluid to the necessary temperature. See, for example, Langson U.S. Pat. No. 7,637,108 (“Power Compounder”) which is hereby incorporated by reference.
As a result of the transfer of waste heat energy from the combustion engine to the ORC system, these types of prior art ORC systems serve two functions. They convert this waste heat energy, which would otherwise be lost, into productive power; and they simultaneously provide a beneficial, and sometimes a necessary, cooling or condensation function for the combustion engine. In turn, the ORC system's shaft output power has been used in a variety of ways, such as to drive an electric power generator or to provide mechanical power to the combustion engine, a pump, or some other mechanical apparatus.
ORC systems can extract as much useful heat energy as they can utilize from one or more waste heat sources (often referred to as the “prime mover”), but owing to various physical limitations they cannot convert all available waste heat to mechanical or electric power via the expansion process discussed above. Similar in some respects to the cooling requirements of the prime mover, the ORC system requires post-expansion cooling (condensation) of its working fluid prior to repressurization of the working fluid by the system pump and delivery of the working fluid to the heat exchanger. The heat energy lost in this condensation process, however, represents wasted energy which detracts from the overall efficiency of the system.
Prior art ORC systems capture a portion of the waste heat energy from either the exhaust gas flow or jacket cooling water, or a combination of both, from a prime mover but must discard a portion of the waste heat energy that might otherwise be captured and converted into useful mechanical and/or electrical energy. Some heat energy is distributed within the internal processes of the prior ORC systems, and this heat energy must be recaptured or it will be lost, thereby decreasing efficiency. For example, the prior art includes systems that utilize superheated fluids, including water, and the recuperation process to increase efficiency (see, for example, Kaplan, US 2010/0071368). This approach recaptures heat energy that would otherwise be lost in the post-expansion fluid during condensation and redirects that energy back to the energy transfer components (vaporizers), which heat the system's working fluid.
The prior art also includes, for example, the use of multiple expanders with multiple heat sources (Biederman, US2010/0263380), cascaded expanders (Stinger, U.S. Pat. No. 6,857,268), and other ORC system configurations with multiple working fluids (Ast, 2010/0242476). These types of systems, however, each add structure and processing to the basic ORC cycle in a fashion that consumes or wastes heat energy that could otherwise be utilized in an ORC cycle. These additional structures also add cost to the systems.
Exacerbating the situation is the fact that these and other prior art systems require the use of high grade waste heat. For example, the expanders typically used in these systems require superheated (other than wet) working fluid. As a result, their input temperature requirements are such that high temperature waste heat is required to properly drive the systems.
Further, these and other references teach the use of additional components, including intermediate heat exchangers to transfer heat energy from one portion of the system to another, including between ORC processes that use separate working fluids of possibly different compositions. Such intermediate components add cost and cause the system to operate at reduced efficiency compared to what can be attained without them.
Further, the use of cascaded heat transfer subsystems necessary to accommodate multiple working fluids decrease the exergy, or the heat energy, recovered from the prime mover that is available for use by the ORC. These types of heat transfer subsystems also increase the cost, complexity, and size of the ORC waste heat recovery system while decreasing reliability and requiring greater maintenance.
Some prior art combined prime mover/ORC engine applications have utilized heat generated by the ORC condensation process in a conventional ORC system condenser while simultaneously providing power (electrical and/or mechanical) for various purposes. Combined heat and power (“CHP”) ORC systems have typically fulfilled a secondary purpose by using a portion of the heat energy from the prime mover and/or heat energy remaining in the post-expansion working fluid. FIG. 5 depicts a prior art ORC system including combustion engine heat energy output port 501 and condenser heat energy output port 502.
In one prior art ORC application, residual heat extracted from a dedicated ORC condenser during the cooling of post-expansion ORC working fluid at condenser heat energy output port 502 is used to provide domestic hot water, radiant heating, or both. This process uses a conventional ORC condenser system well known in the art. The energy flow of one such application is depicted in the block diagram of FIG. 6. In this application, a heat generating engine 601 is operatively coupled to electric generator 602 and provides waste heat energy 603 to the ORC system 604. In turn, the ORC system 604 is operatively coupled to drive electric generator 605. Heat energy from the prime mover 601 is delivered to heat energy output port 501 and, in some prior art systems, is extracted to a first heat energy input port 606 (such as for radiant heating); in addition, heat energy from the ORC condenser is delivered to a second heat energy input port 607 (such as for hot water heating). In those ORC systems known by the applicants, the utilization of residual heat from the post-expansion working fluid is intentionally extracted from the system but is not utilized for further system optimization of the prime mover or, for example, for heating a production material such as microorganisms to generate biofuel.
As noted above, screw and twin screw expanders have long been utilized in many applications in the prior art. Certain of these types of expanders have long been capable of operating with wet (i.e., non-superheated) working fluid. As a result, these types of expanders have also long been utilized with heat sources and working fluid temperatures well below the comparable temperatures provided by high temperature heat sources and the superheated working fluid developed in the associated ORC and its expander as a result.